Samantha Power – the US ambassador to the UN and a chief interventionist in the Obama White House – spoke at a Washington think tank Friday, laying out the case for taking military action against Syria.

Washington — One of the chief interventionists of the Obama White House offered a preview Friday of the arguments the president will make when he addresses the American people Tuesday evening on the case for taking military action against Syria over the use of chemical weapons.

Samantha Power, the US ambassador to the United Nations and one of President Obama’s aides who argued successfully in 2011 for US intervention in Libya, told a small Washington audience that “chemical weapons are different.” The consequences of letting the worst instance of their use in decades go unpunished, she said, would stretch well beyond Syria or even the Middle East.

“We cannot afford to signal to Iran and North Korea that the world is unable to stop the proliferation of weapons of mass destruction, and unwilling to act against [their] use,” she said.

Ambassador Power spoke at the Center for American Progress (CAP), a Washington think tank that is generally friendly toward the policies and goals of the Obama administration. The human rights advocate and member of the president’s first-term national security team was greeted with polite applause for her speech, while outside a small group of protesters shouted, “No war!” and “Shame on the Center for American Progress for hosting a warmonger!”

Power acknowledged that the American public is highly reluctant to enter another Mideast conflict and is worried about the risks that come with getting involved. But expressing a sentiment that Mr. Obama and other administration officials have been underscoring all week, she said, “The risks of not acting are far greater than the risks of taking targeted action” against the regime of Syrian President Bashar al-Assad.

Power’s speech struck more than one listener as something of a practice run for the address Obama plans to deliver Tuesday evening. That speech will be part of his effort to win over a Congress that by all reports is hearing mostly “no” from constituents.

“She pretty much laid out the way Obama is going to do this, the points he’ll make to try to convince the public that this thing is necessary,” says Lawrence Korb, a national security expert at CAP and a former Pentagon official during the Reagan administration.

Those points include, “We tried the diplomatic route and it didn’t work, there will be no boots on the ground, letting Assad get away with using chemical weapons will undermine our ability to stop them elsewhere, that kind of thing,” Mr. Korb says.

To that list he adds, in another paraphrase of Power, “When we launch these strikes, whatever they might be, this will convince Assad he can’t win – and that could lead to some kind of diplomatic solution.”

But in her speech, Power was careful to emphasize time and again that the focus of the military action the president believes the United States should take is Assad’s apparent use of chemical weapons.

“The limited military actions [envisioned] are not designed to resolve the Syrian crisis,” she said. The “limited military measures” are meant to “deter Assad from using these chemical weapons again” and to “degrade” his ability to use chemical weapons.

Offering one hint of an expanded purpose for the strikes, Power added that they would also aim to “degrade [Assad’s] ability to strike the civilian population by conventional means.”

The top US diplomat at the UN reviewed the history of Russian obstruction in the UN Security Council of any action on Syria since the civil war started more than two years ago. To those critics of military strikes who advocate more UN diplomacy first, Power responded, “Yes! We would if we could – but we can’t.”

She called it “naive” to think that Russia is “about to change its position and allow the Security Council to play its rightful role.” Anyone thinking otherwise, she said, need only recall the Council’s inability to respond with even the simplest steps after the Aug. 21 chemical weapons attacks that the US says killed more than 1,400 people in the Damascus suburbs.

“The Security Council could not even agree to put out a press statement,” she said.

Power spoke the same day Russia’s ambassador to the US, Sergey Kislyak, told another Washington think tank audience that Russia’s primary concerns about possible US action in Syria are the impact it would have on already delicate regional stability, and what it would mean for international order.

“We are very much concerned about steps the US can take and the consequences they would have on the ground and in their impact on international law,” he said, speaking at the Center for the National Interest, an organization that advocates a realist US foreign policy.

Ambassador Kislyak insisted that, contrary to the picture painted by the US and other Western powers, “We are serious about upholding international norms.” But he pushed back at the Obama administration’s targeting of Russia over what it calls obstructionism in the Security Council, saying the West does the same thing.

He cited a report that Russia presented to the Security Council claiming to document a chemical attack by opposition forces in March in Aleppo, Syria’s largest city. The Council did not take up the report or act on it, he said.

“And now the US has decided to decide everything on its own,” he said.

The American public and Congress are reluctant to sign on to military strikes, says Korb of CAP, because what they would accomplish is not any more clear than where action could lead.

“There are no good options, because no matter what you do there are risks,” he says.

Despite that, Korb says he believes that in the end, Congress will vote to authorize military strikes.

“People forget, but the vote on the [1991] Persian Gulf War was very close. The Senate voted 53 to 47 in favor,” he says. “Right up to the end, there were people in the administration who thought they were going to lose.”